The present invention relates to processing of wafers and, more particularly, to systems and methods for quality control in deposition processing.
A variety of deposition techniques are available for processing of wafers to fabricate semiconductor devices. Rapid thermal processing (RTP), for example, is widely used in semiconductor fabrication applications in which rapid temperature cycling is necessary or desirable. Examples of common rapid thermal processing applications include annealing, oxidation, and nitridation. A rapid thermal processing chamber typically includes a housing with a support for a wafer to be processed, one or more heat sources, such as lamps, that generate radiation to heat the wafer, and a reflector plate that forms a reflective cavity for more effective heating.
Temperature uniformity is a critical factor in the quality of the rapid thermal process. Different areas of the wafer can exhibit different energy absorption or emissivity characteristics. Moreover, the spatial heating profile of the heat source can be somewhat nonuniform. Consequently, the rapid thermal process can produce significant thermal gradients across the surface of the wafer. Excessive thermal gradients can result in structural damage to the wafer, directly impacting device yield and quality. With flood lamp heat sources, it is often difficult to control cross-substrate temperature. With zoned heat sources, however, temperature can be spatially controlled to more effectively minimize thermal gradients across the wafer.
For spatial control of cross-wafer temperature, particularly with zoned heat sources, a rapid thermal processing chamber typically incorporates a temperature sensing device. The temperature sensing device may include, for example, an array of temperature sensor probes such as pyrometers. The temperature sensing device senses the temperature of the wafer, often at several positions, during the heating cycle. Temperature signals generated by the temperature sensing device are processed to generate control signals for the heat source.
Accordingly, the accuracy of the temperature signals provided by the temperature sensing device is important for effective control of the heat source, and therefore is a significant factor in device fabrication quality and yield. Any inaccuracies in the temperature measurement can undermine the effectiveness of the temperature control function, opening the door for temperature gradients that can damage the wafer being processed.
The present invention is directed to a system and method for detecting wafer fragments in a wafer processing apparatus. The system and method can be useful in maintaining the accuracy of the temperature control function of a wafer deposition process. In particular, the system and method facilitate the detection of wafer fragments in a deposition processing chamber. Such wafer fragments can adversely affect the accuracy of temperature measurements within the chamber.
The system and method are particularly useful in controlling the quality of a rapid thermal process given the stringent temperature requirements of such a process. The system and method may find ready application, however, in a variety of wafer processes in which the presence of fragments is a concern, and therefore are not limited to rapid thermal processing. Nevertheless, for purposes of illustration, reference will be made to the characteristics of the rapid thermal process in describing an embodiment of the present invention.
During a rapid thermal process, fragments can break away from the wafer and fall onto the temperature sensing device, or onto areas between the device and the wafer such as the reflector plate. The wafer fragments can compromise the accuracy of the temperature signals generated by the temperature sensing device. With pyrometer probes, for example, wafer fragments can fall onto the reflector plate at positions generally coincident with the probes. The wafer fragments can attenuate or otherwise interfere with the infrared radiation received from the wafer.
This interference can undermine the accuracy of the temperature measurement signal generated by the probes, and ultimately degrade the temperature control function across the surface of the wafer. If the temperature control function is ineffective, excessive temperature gradients can result in damage to the wafer, reducing device yield and degrading device quality. Also, the effects of the wafer fragment can persist for subsequent wafers processed within the rapid thermal process chamber unless the fragment is cleared away from the reflector plate.
To alleviate the effects of wafer fragments, in accordance with an embodiment of the present invention, the presence of a wafer fragment over a temperature sensing device such as an array of probes is detected. The presence of a wafer fragment can be detected, for example, using machine vision techniques. An image acquisition device acquires an image of at least a portion of a reflector plate. The acquired image is analyzed to detect the presence of a wafer fragment.
Detection of a wafer fragment allows the rapid thermal process to be stopped so that the fragment can be cleared away. For example, upon detection of a wafer fragment, an advisory can be generated. The advisory may serve as the basis for automated halting of the process, or as a notification to a human operator for manual intervention. By removing the fragment, a potential source of error in the temperature measurement generated by the temperature sensing device can be eliminated before the next wafer is processed. In this manner, the accuracy of the temperature measurement can be maintained, thereby reducing the possibility of excessive temperature gradients that can lead to device yield and quality deficiencies.
In one embodiment, the present invention provides a system for detecting the presence of a wafer fragment in a deposition process chamber, the system comprising an image acquisition device disposed to acquire an image representative of an area between a temperature sensing device and a wafer processing position within the deposition process chamber, and a processor that analyzes the acquired image to detect the presence of a wafer fragment within the area.
In another embodiment, the present invention provides a rapid thermal processing system comprising a process chamber, a wafer mount disposed within the process chamber, a heat source disposed to heat a wafer in the wafer mount, a reflector plate disposed on a side of the wafer mount opposite the heat source, one or more temperature sensor probes disposed proximate to the reflector plate, a viewport allowing visual access to the process chamber, an image acquisition device disposed to acquire an image via the viewport, the image being representative of at least a portion of the reflector plate within the process chamber, and a processor that analyzes the acquired image to detect the presence of a wafer fragment on the reflector plate.
In a further embodiment, the present invention provides a method for detecting the presence of a wafer fragment in a deposition process chamber, the method comprising acquiring an image representative of an area between a temperature sensing device and a wafer processing position within the deposition process chamber, and analyzing the acquired image to detect the presence of a wafer fragment on the surface.
In each of the above embodiments, the acquired image can be analyzed by comparison to a reference image representative of a reflector plate that is substantially free of wafer fragments. In this case, an advisory can be generated in the event the acquired image deviates from the reference image by more than a threshold deviation representative of the presence of a wafer fragment. Alternatively, the acquired image can be analyzed by identifying an optical density contrast change at a position on a surface of the reflector plate, and determining whether a wafer fragment is present based on the contrast change. In either case, the presence of a wafer fragment can be detected using known image processing techniques employed in existing machine vision systems.
Such machine vision techniques can be used to detect the presence of a wafer fragment and, if desired, quantify the size of the fragment. With size information, a size filter can be applied to the detection process. In particular, generation of an advisory can be triggered based on the presence of a wafer fragment that exceeds a size sufficient to adversely affect the temperature control function. In this manner, the presence of very small fragments that are considered unlikely to cause temperature control problems can be discounted based on selection of a size threshold by the process operator.
Other advantages, features, and embodiments of the present invention will become apparent from the following detailed description and claims.